AR Sponsor
Posted: 7/18/2004 7:24:21 PM EDT
| Have a BFI RR that I have put several hundred rounds through both semi and full auto. It has run perfectly in the past. A week ago it started to mal function in full-auto. The action cycles after firing a round, fully chambering the second round. The second round fails to fire and has just a slight primer indent. NO signs of broken or worn parts. Have looked through 3 military manuals trouble shooting sections with no succcess. In looking as close as I can, would guess some how the hammer is releasing early, following the bolt home which lessons its force as it hits the bottom of the bolt and then the firing pin a split second later. BUT, every thing appears OK and seems to function OK when cycled by hand: hammer seems to drop as the bolt is rotating into battery. Any ideas would be appreciated. Bruce |
|
Let me guess, You built the RR into a Carbine using a standard aftermarket kit and the rifle ran fine. Then as everything self polished threw wear, the upper is slick as shit and the carrier is sliding threw the upper with the greatest of ease, you started to have a problem (maybe even changed ammo type). Also, the recoil spring has taken a set, so now the B/C is really moving without any surface binding (full Auto cycle rate is higher than when you first built the RR).* BOLT/CARRIER BOUNCE!!!!!!!! The carrier is bouncing back off the barrel extension as the hammer is making contact with the firing pin. This bounce causes the carrier hood to take the hammer strike, instead of the hammer. The slight indent of the primer is caused by the floating firing pin, and not by the hammer. The solution if the Auto sear release gap is between .080 and .095 if to just chuck the light weight buffer in the trash and buy a "H" buffer to control the Bolt/ Carrier, and keep it from bouncing open at lock up/hammer release. * Did I guess right????  |
|
Tweak--Gunbert--Dano523 Appreciate your feed back. A little more info on the Gun and problem. Was purchased as a Carbine 14.5 bbl with older style telestock. Haven't changed ammo using Malaysian surplus 4-82. BUT bought a new complete 11.5 upper and new style telestock from Bushie. (Wanted the shorter bbl and original telestock was buggered up). Put both on including a new buffer (carbine and not an H) and spring, again all from Bushie. First 20 rds, with the new upper assembly/stock/buffer/spring were fired semi with NO failures. Handed to my son for the first FA burst. As best as we can remember fired 2 or 3 rds FA then FTF. Cycled action round in chamber with slight dent in primer (like firing pin bounce, as Dano523 suggested). Tried several more times with same result. Couple days later put complete 14.5 back on with IT'S original spring and buffer BUT still have the new rear telestock tube. All attempts to fire in FA result in single firing and chambered rounds with slight primer dents. Changed the spring and buffer, still same result FTF on the second round. With your info and suggestions will check with drill bits for correct timing and try a H buffer. Hammer and trigger springs seem to be very stiff. For sure the new rear telestock/tube is the one potential problem, have NOT tried changing back to the old one. Will check and report back. Glade to have some knowlegeable input will report back as I can. thanks again. Bruce |
Is this on one uppers, or just a new upper/carrier? Try them all so we can get an idea on to either replace parts or modify the existing parts (conversion on the semi receiver to RR may have resulted in the auto sear hole being drilled a tad too far back and the original conversion parts modified to work with the new RR/sear location). If all uppers/carrier release Plus .100, then hammer and auto sear may need to be replaced. Or, if all the FCG parts are new, then you will need to modify the carriers to retard the sear release. Advancing a sear release is removal of metal from the auto sear hammer tab, retarding release is done by milling the carrier bottom cut back to trip the sear latter on the forward stroke. Since were talking about a $10,000 RR, it may be time to find a good smith to get all the uppers timed correctly to the RR. A few hundred dollars of shop charges is a lot easier to swallow than KB’ing a RR if you get the timing all screwed up. |
|
Thanks, Dano. In my case, it was a theoretical question. The only ways I could picture a fix for an early trip was to (depending on how much the gap is off): A. Cut back the collar on the carrier B. Thin the carrier paddle of the autosear a little C. Thicken the hammer paddle of the autosear The direct opposite of the above would serve to resolve a late trip. If the case arose, I would personally prefer one of the above fixes than having someone muck with the lower itself unless I was extremely confident in their ability as a smith, but I wanted to check to see if other options were possible. |
AR Sponsor